This invention concerns acid catalysts for coating compositions which are stable for long shelf life and can be cured at relatively low temperatures.
In recent years there has been substantial work in the field of acid catalysts for low temperature cure (80.degree.-110.degree. C.) finishes based on hydroxyfunctional resins and "melamine" crosslinkers. Several patents have appeared such as Canadian No. 1,201,239, U.S. Pat. No. 4,477,618 Singer et al. (1984), and British No. 1,413,054 (1975) and 1,361,929 (1974), focusing on making blocked acids with low "unblocking" temperatures.
A study of L. Hill (J. Coatings Tech Vol 53, No. 675, 1981) did show that a blocked acid catalyst with both large enthalpy and entropy of activation on deblocking would be most suitable for maximizing stability and minimizing cure temperatures. His theoretical calculations showed that it would be unlikely to combine adequate shelf stability with low temperature reactivity, taking into consideration only the above mentioned kinetic effects with the unimolecular deblocking reaction as rate controlling step.
Those calculations apply for homogeneous systems in which there is no phase separation on cure and which do not take into consideration diffusion effects due to viscosity changes during crosslinking and film formation. The use of strong acids (blocked or unblocked), such as phosphoric acid and sulfonic acid derivatives, for polyol-melamine based coatings is well known and described in literature. The problems, however, remain the poor balance of shelf stability--cure temperature and curing time--overall film properties, including humidity and corrosion resistance)--compatibility with polyol and melamine crosslinker. The use of polymeric type strong acid catalysts in which 2-acrylamine-2-methylpropane sulfonic acid (AMPS) is copolymerized with other monomers to improve those properties, has been described in many patent applications.
U.S. Pat. No. 3,711,449 --Brendley (1973) teaches interpolymers of 0.2-1.0% 2-acrylamide-2-methylpropane sulfonic acid (AMPS) with other acrylates or methacrylates.
U.S. Pat. No. 3,898,037--Lange et al. (1975) teaches acrylamido-sulfonic acid polymers such as AMPS which may be copolymerized with other acrylics, and their use in corrosion inhibition.
U.S. Pat. No. 4,177,178--Das et al. (1979) teaches the use of AMPS copolymerized with other acrylics including long chain acrylics such as stearyl methacrylate, with a molecular weight of 15,000 to 100,000. These are said to be used to make thermosetting automotive topcoat paints.
U.S. Pat. No. 4,001,150--June et al. (1977) teaches the use of phosphoric or sulfonic esters, including AMPS, copolymerized with acrylics such as methacrylic acid, for use as an electroconductive resin.
U.S. Pat. No. 2,914,499--Sheetz (1959) teaches the use of various acrylic esters of sulfonic acid in emulsion polymerization.
U.S. Pat. No. 4,008,293--Maska et al. (1977) teaches the preparation of crosslinkable coating compositions containing, for instance, AMPS, as an internal crosslinking catalyst.
The problem, however, with many polymeric types of strong acid catalysts can be insufficient mobility and compatibility with the polyol-melamine binder due to viscosity change on crosslinking and film formation. Polymeric types of strong acid catalysts can also impart a strong viscosity rise due to hydrogen bonding with the other paint constituents which has a negative effect on the solids content of the final formulation. Another problem one can have with both monomeric and polymeric types of strong acid catalysts in pigmented applications is absorption on pigments and extenders which reduces the activity of the acid catalyst initially and on storage. To improve above properties it would therefore be an advantage to have the strong acid catalyst in a different phase on storage temperature, becoming available at the specific curing temperature.
Non-aqueous dispersions (NAD) are a class of polymer dispersions in which a high molecular weight (or crosslinked) core polymer is sterically stabilized by another polymer grafted or absorbed onto it. The solubility difference between grafted and/or absorbed and core polymer should be sufficient to keep the dispersion stable. Numerous patent applications describe the technology of making NAD. One could imagine the strong acid copolymerized with the core polymer composition to have it in a separate phase on storage and to become available on crosslinking due to temperature-solubility change. The problem, however, remains the polymeric nature which can give compatibility problems with the other binder constituents on crosslinking and film formation. Another problem remains the strong viscosity change and instability, since hydrogen bonding between strong-acid-in-core polymer of the NAD and hydroxy of the polyol drives the strong acid groups to the particle surface It would therefore be ideal to have a semi-crystalline dispersed monomeric or oligomeric type strong acid catalyst which becomes soluble at the specific curing temperature due to a narrow softening or melting range in which the catalyst becomes solubilized in the other paint components.